The invention relates to a suspension system for suspending a low temperature tank in an outer shell, whereby the connection between the outer shell and the low temperature tank is accomplished by a plurality of straps made of fiber compound materials.
It is well known to use fiber compound materials for suspension systems of the above type because of the desirable material characteristics of such materials. The connecting or securing straps are formed as integral single piece members and preferably the fiber orientation is uni-directional relative to the longitudinal extension of the strap or straps. Among the material characteristics desirable for the intended purpose, are the following: the high material strength, the stiffness, the low weight as well as the small creep rates under load. In addition to the large mechanical loads the connecting straps must also satisfy enormous thermal load requirements. On the one hand it is required that the straps have a high resistance to heat conduction so that there will not be formed a thermal bridge between the outer shell and the low temperature tank when the latter is in its cold condition. On the other hand the straps must remain under substantially uniform tension loads under all operating conditions to assure a precise fixing of the low temperature tank inside the outer shell. This operating condition must be assured in spite of large temperature variations of the inner, low temperature tank. This temperature variation may range from room temperature when the tank is empty to extremely low temperatures when the tank is filled, for example, with a liquified gas. It has been found that prior art suspension straps made as single piece, integral components, are not capable to satisfactorily handle these thermal loads although the mechanical strength and stiffness of prior art suspension systems are satisfactory.
Where prior art suspension straps are made of glass fibers it is possible to make sure that the connecting straps have a sufficiently high resistance against thermal conduction. However, the changes in strap length due to thermal expansion in response to the temperature changes between the lowest temperature condition and room temperature when the tank is empty, are so large that unpermissible conditions occur. Thus, in the cold condition the straps may be exposed to unpermissible excess tensions. On the other hand, when the tank warms up to room temperature a precise suspension or fixing of the tank is not assured any more due to the excessive increase in strap length. When the integral, single piece connecting straps are made of carbon fibers, it is possible to keep the tension load variations sufficiently small throughout the entire temperature range between the room temperature and the lowest possible temperature of the inner, low temperature tank. However, due to the relatively large heat conduction coefficient of carbon fiber material on excessive heat flow along the connecting straps cannot be avoided when the inner tank is in its low temperature, filled condition.
Even where different fiber types are used for the connecting straps, it has been impossible heretofore to avoid, depending on the type of fibers, either an excessively high tension load variation between the room temperature and the low temperature level of the inner tank or to avoid an excessively large heat flow along the connecting straps in the cold condition of the inner tank.